fix incorrect ptrdiff_t type on mips

arch/mips/bits/alltypes.h.sh

@@ -18,7 +18,7 @@ union \1 \2;\
 
 TYPEDEF unsigned size_t;
 TYPEDEF int ssize_t;
-TYPEDEF long ptrdiff_t;
+TYPEDEF int ptrdiff_t;
 
 TYPEDEF __builtin_va_list va_list;